1. Technical Field of the Invention
The present invention relates, generally, to compositions and methods for the treatment of cancer and infectious disease. More specifically, this invention provides alternative reading frame (ARF) polypeptides, conjugates, and fusion proteins; polynucleotides encoding (ARF) polypeptides, conjugates, and fusion proteins; and antigen presenting cell (APC) and dendritic cell (DC) based compositions and methods that employ ARF polypeptides and polynucleotides which compositions and methods are useful in the treatment of cancer and infectious disease.
2. Description of the Related Art
The central dogma of molecular biology provides that genomic DNA is transcribed into mRNA and mRNA is translated into protein. According to recent estimates, the human genome encodes approximately 30,000 to 100,000 mRNAs that are translated into proteins that, in total, comprise the proteome. Harrison et al., Nucl. Acids Res. 30:1083-1090 (2002).
The mechanism by which eukaryotic mRNA is translated into protein is well established. Through the process of ribosome-scanning, a 43S pre-initiation ribosomal complex assembles on the 5′ mRNA CAP and migrates in a 5′ to 3′ direction along the 5′ untranslated region (UTR) in an ATP-dependent process. When the 43S complex encounters an initiator AUG codon within the proper context (normally the first AUG 50 to 100 nucleotides downstream of the CAP), it pauses for a time sufficient to permit the association of the 60S ribosomal subunit to create the ribosomal initiation complex that commences translation in the normal, rf0, open reading frame. See, Kozak, J. Cell Biol. 108:229-241 (1989); for a review, see also, Gray et al., Annu. Rev. Cell Dev. Biol. 14:399-458 (1998).
The ribosomal machinery of a eukaryotic cell can produce errors in translating mRNA that result in aberrant translation products such as alternative reading frame (ARF) polypeptides. ARF polypeptides are translational products encoded in reading frames rf1 and rf2 that are shifted by one or two nucleotides, respectively, from the normal rf0 reading frame. Because their expression is low, ARF polypeptides are biologically invisible to the cell; however, due to the exquisite sensitivity of the cytotoxic T-cell (CTL) response, ARF polypeptides may be immunogenic at very low copy numbers.
ARF polypeptides are produced entropically due to errors inherent in protein synthesis. Yewdell et al., J. Immunol. 157:1823-1826 (1996). Mechanisms by which translational errors produce ARF polypeptides include: (1) initiation of translation in the 5′ UTR (Uenaka et al., J. Exp. Med. 180:1599-1607 (1994)); (2) frame-shifting of the initiation complex at the normal rf0 AUG codon one base (rf1) or two bases (rf2) forward or one base (rf2) or two bases (rf1) backward; (3) scan-through the normal rf0 AUG codon and formation of an initiation complex at a downstream codon (Bullock et al., J. Exp. Med. 186:1051-1058 (1997)); (4) formation of an initiation complex at an internal ribosome entry site (IRES) located 3′ to the site of normal cap-dependent ribosomal entry (Nanbru et al., J. Biol. Chem. 272:32061-32066 (1997); (5) frame shifting of the ribosome through random and programmed frameshifts from rf0 to rf1 or rf2 reading frames (Elliott et al., Eur. J. Immunol. 26:1175-1179 (1996); Rom et al., Proc. Natl. Acad. Sci. 91:3959-3963 (1994); Farabaugh, Annu. Rev. Genet 30:507-528 (1996)); (6) formation of initiation complexes at rf1 or rf2 codons other than AUG, such as, for example, ACG or CTG (Shastri et al., J. Biol. Chem. 270:1088-1091 (1995); Malarkannan et al., Immunity 10:681-690 (1999); and Ronsin et al., J. Immunol. 163:483-490 (1999)); (7) ribosomal skipping of mRNA segments (Herr et al., Annu. Rev. Biochem. 69:343-372 (2000)); (8) ribosomal suppression of termination codons and subsequent translational readthrough (Bullock et al., J. Exp. Med. 186:1051-1058 (1997)); and (9) synthesis of ARF polypeptides in reading frames 3, 4, and 5 (i.e. rf3, rf4, and rf5) resulting from the translation of antisense strands of genes that are expressed through transcription from cryptic promoters (Van den Eynde et al., J. Exp. Med. 190:1793-1799 (1999)).
Upon translation, ARF polypeptides are likely to act as substrates for the ATP-dependent TAP transporters, to be translocated into the lumen of the ER, and to be loaded onto major histocompatibility complex (MHC) class I molecules bound for the cell surface of the antigen presenting cell (APC). The ARF polypeptide is presented within the context of an MHC class I molecule to naïve CD8+ cytotoxic T-cells (CTL). This stimulates the clonal expansion of ARF-specific CTL capable of identifying and eliminating cells expressing the ARF polypeptide. For a review, see, Rock et al., Annu. Rev. Immunol. 17:739-779 (1999).
The 5′ UTR of many human genes helps to regulate translational initiation of the structural gene. Alternative 5′ UTR initiation codons is one mechanism by which translation initiation is regulated. The 5′ UTR of human genes is a likely rich source of Arf peptides due to this regulatory mechanism. For example, JunD mRNA, which translates in a cap-dependent manner, initiates at two in-frame AUGs, yielding a 39 and 34 kDa protein. JunD mRNA encodes an out-of-frame AUG that directs translation of a short Arf peptide as well as three non-AUG codons also able to support translational initiation, in frame ACG and CUG codons down stream of the rf0 AUG, and an out-of-frame CUG found in the 5′ UTR that should generate an Arf peptide. Short et al., J. Biol. Chem. 277:32697-705 (2002). These codons function to cumulatively suppress 34 kDa translation. The 5′ UTR of eIF4GI mRNA contains an out-of-frame AUG that acts to regulate expression of eIF4GI and produce an Arf peptide. Byrd et. al., Mol. Cell. Biol. 22:4499-511 (2002).
Other mechanisms can account for Arf production in human genes. For example, C- and L-Myc (Joplin et. al., RNA 10:287-98 (2004) and Cencig et. al., Oncogene 23:267-77 (2004)) angiotensin II type 1 receptor (Martin et. al., Mol. Cell. Endocrinol. 30:51-61 (2003)); and HSP70 (Rubtsova et. al., (2003)) initiate translation via 5′-UTR IRES sequences in a cap-independent manner. Alternative splicing in the 5′ UTR will also give rise to novel peptides.
Tumor infiltrating lymphocyte (TIL)-derived ARF-specific CD8+ CTLs have been identified in melanoma and renal cell carcinoma patients. Wang et al. J. Exp. Med. 183:1131-1140 (1996); Moreau-Aubry et al. J. Exp. Med. 191:1617-1623 (2000); Rosin et al. J. Immunol. 163:483-490 (1999); and Probst-Kepper et al. J. Exp. Med. 193:1189-1198 (2001). Only a small number of tumor antigens have been identified by TIL-derived CTLs; however, a substantial number of those identified arise from ARF polypeptides. Rosenberg, Immunology Today 18:175-182 (1997). For example, an ARF polypeptide has been identified using a lymphocyte clone from a patient exhibiting complete regression of melanoma metastases. Rosenberg et al. J. Immunol. 168a:2402-2407 (2002).
While the existence of alternative reading frame polypeptides has been described, it has not been appreciated that ARF polypeptides may be employed in compositions and methods for stimulating a protective immune response specific for cancer and/or infectious disease antigens.